Continuous glucose monitoring (CGM) involves repetitive measurement of physiological glucose concentration to enable close monitoring and timely correction of problematic blood sugar patterns of patients with diabetes mellitus, thereby reducing the risk of diabetes-related complications and ultimately allowing closed-loop blood sugar monitoring and insulin administration. Commercially available CGM sensors that use electrochemical methods are currently hindered by limitations such as low accuracy (especially at hypoglycemic glucose concentrations), poor stability, and long lag times. We aim to address these issues by developing a subcutaneously implantable affinity microsensor for continuous monitoring of glucose in interstitial fluid. The microsensor, created using microelectromechanical systems (MEMS) technology, will have a miniaturized, flexible design to realize differential measurement of affinity binding of glucose to a synthetic, biocompatible hydrogel. Affinity binding, in which glucose binds specifically and reversibly to the hydrogel without glucose- consuming chemical reactions commonly found in existing glucose sensors, affords high stability. MEMS- based differential measurement of affinity binding enables rapid, accurate determination of glucose concentration, in particular in the hypoglycemic range, in the face of nonspecific disturbances. These functions are realized in a miniaturized, flexible design, which minimizes the effects of device-tissue interactions. In design, the microsensor resides on a flexible substrate and is integrated with a glucose-binding (sensing) hydrogel and a glucose-insensitive (reference) hydrogel. With an active sensing region hundreds of micrometers in size, the device is implanted (via a small needle) beneath the skin in the abdominal region. During operation, glucose molecules in tissue rapidly enter the microsensor and bind reversibly to the sensing hydrogel, whose dielectric properties change accordingly. Meanwhile, the reference hydrogel's dielectric properties change only with nonspecific disturbances (e.g., temperature). Thus, differential dielectric measurement allows accurate determination of the glucose concentration in the interstitial fluid. The direct goal of this project is to develop the differential affinity microsensor for percutaneously implanted operation over a period of 5-7 days with a high level of stability and accuracy. The specific aims include (1) functional hydrogel synthesis, (2) device design and fabrication, and (3) hydrogel and device characterization in vitro and in vivo. The device will in the future be further developed to allow long-term (months or longer) implanted operation, and be included in an artificial pancreas to enable closed-loop glucose control.